1. Field of the Invention
The invention relates in general to a method of a phase shift mask. More particularly, the invention relates to a three-phase phase shift mask to resolve the corner round problem occurred in an unexposed edge area caused by diffraction or scattering of light.
2. Description of the Related Art
As the integration of an integrated circuit is demanded higher and higher, the design is developed toward a direction of further shrinking the devices and the circuit. The photolithography technique plays one of the most important roles for the shrinkage. For examples, the sizes of any structures related to a metal-oxide semiconductor (MOS) such as a thin film pattern and the dopant area are basically determined by this technique. Thus, whether the integration of semiconductor industry can be further developed down to, over even under, a linewidth of 0.15 micron, is determined by the development of photolithography technique. According to the great demand, methods to enhance the resolution of photomask such as using optical proximity correction (OPC) and phase shift mask (PSM) have been proposed.
The method of optical proximity correction is to eliminate the deviation in critical dimension (CD) caused by the proximity effect. When a light beam is incident on a wafer through the pattern of a photomask, the light beam is scattered so that the area of the wafer spotted by the light is enlarged. On the other hand, the light beam may be reflected from the semiconductor substrate of the wafer to cause an interference with the incident light beam. As a result, a double exposure is caused to change the exposure degree of the wafer. The proximity effect is even more obvious when the critical dimension is close to the wavelength of the incident light.
Referring to FIG. 1A to 1D, a conventional method of optical proximity correction is drawn. In FIG. 1A, a photomask 100 having a pattern of three rectangular masking areas are shown and denoted as 105, while the rest area of the photomask is transparent and denoted as 110. The substrate material of the photomask 100 is typically glass or quartz that forms the transparent area 110. The masking areas 105 are typically made of a layer of chromium (Cr). In FIG. 1B, when a light beam is incident on the photomask 100, a wafer substrate 120 underneath would have three dark regions 125 and a light regions 130 on a substrate 120.
As shown in FIGS. 1A and 1B, the masking areas 105 are in rectangular shape, however, the pattern transferred onto the wafer substrate 120 becomes dark regions 125 with rounded corners and smaller dimensions. Patterns or masking areas in other region that is not shown in this figure may be distorted or deformed in other form. For example, when the mask regions of the pattern are designed close to each other, after exposure, the patterns transformed into the wafer substrate might merge with each other or deviate from the original pattern.
In general practice, to compensate the above deformation of patter, at the corners or edge of the masking areas 105, assistant features such as serifs 150 and 155 at the corners and along the edge as shown in FIG. 1C. the serifs 150 at the corners are added to resolve the problem of rounded corner, while the serifs 155 along the edge are added to restore the desired dimensions of the pattern. As shown in FIG. 1D, using this method, the fidelity of the pattern transferred from the photomask 100 to the wafer substrate 120 is very much improved. The dark areas 125a has a much less rounded corners, while the dimensions of these areas 125a are closer to those 105 on the photomask 100.
However, when the distance between patterns is further reduced or the critical dimension of is further shrunk to lower than 0.1 microns, this method meets its bottleneck. That is, using this method for compensation or amendment of the patterns, the available spaces or areas for forming or adding the assistant feature such as serifs are too small.